Water is a foreign environment for man: yet with certain mechanical aids he can live and work in this medium for a finite period of time. A major limiting factor is the heat loss which occurs. The first overall objective of this proposed research is to provide an accurate quantitative description of heat exchanges between man and the water. A direct whole body water calorimeter has been constructed and is in operation. We anticipate that by studying subjects in water of different temperatures and measurements of heat balances under several conditions we will develop methods of indirect calorimetry similar to those used for man in air. After this phase of the work is completed we will utilize this method of indirect calorimetry to describe the effects of water velocity on heat loss. We will expand our previous studies of exercise in water to encompass a greater range of oxygen consumption and water temperatures than possible to date. We expect to answer the old riddle about the relative heat losses of man at rest and shivering versus man exercising in the water. The use of the water medium will provide practical and applicable information about man's thermoregulatory capacities.